A combustion engine burns an air and fuel mixture to generate a driving torque. The combustion process generates exhaust gases that are released from the engine into the atmosphere. The exhaust gases contain, among other things, nitrous oxides (NOX), carbon dioxide (CO2), carbon monoxide (CO) and particles. NOX is a collective term used to describe the exhaust gases that consist primary of nitrous oxide (NO) and nitrogen dioxide (NO2). An exhaust gas aftertreatment system processes the exhaust emissions in order to reduce the emissions before they are released into the atmosphere. In one exemplary exhaust gas aftertreatment system, a dosing system injects a reducing agent into the exhaust gases upstream of a selective catalytic reduction catalytic converter (SCR catalytic converter). The exhaust gas and reducing agent mixture react in the SCR catalytic converter, thereby reducing the amounts of NOX that are released into the atmosphere.
One example of a reducing agent is liquid urea, which is commercially available as AdBlue®. This fluid is a non-toxic aqueous urea solution that is used to chemically reduce emissions of nitrous oxides, particularly in heavy diesel-powered vehicles.
The reducing agent reacts with NOX in the SCR catalytic converter to achieve the NOX reduction. More specifically, the reducing agent is broken down and forms ammonia (NH3), which in turn reacts with NOX to form water and nitrogen (N2).
To achieve the described NOX reduction, NH3 must be stored in the SCR catalytic converter. For the SCR catalytic converter to be able to work efficiently, the stored level must be at an adequate level. In further detail, the NOX reduction, or the conversion efficiency, is dependent upon the stored level. The store of NH3 must be maintained in order to maintain high conversion efficiency under various operating conditions. However, as the temperature in the SCR catalytic converter rises, the NH3 level must be reduced accordingly to prevent the release of NH3 (i.e. a surplus of NH3 being released from the SCR catalytic converter), which can lower the conversion efficiency of the catalyst.
In summary, to satisfy more stringent environmental requirements, more and more vehicle manufacturers are using SCR catalytic converter systems to cleanse diesel exhaust gases of nitrous oxides (NOX). This is done by injecting ammonia solution into an SCR catalytic converter, which helps to convert NOX particles into nitrogen and water. The exhaust gas purification strategy should take into account that a sufficient amount of NOX must be converted while avoiding injecting too much ammonia, in view of both fuel economy and environmental considerations.
At least one diesel oxidation catalyst (DOC) is also used in exhaust gas aftertreatment systems, and one or a plurality of diesel particle filters (DPF) are also often coated with a catalytic coating. The purpose of this is in part to generate a sufficient amount of NO2 to achieve passive oxidation of soot that is captured by a DPF. This occurs according to the reaction: C+2NO2→CO2+2NO
In those cases where not all the ammonia has been consumed for the desired reduction in the SCR catalytic converter, it can be stored in the SCR catalytic converter, entrained in the exhaust gases from the SCR catalytic converter or reacted in the SCR catalytic converter to form N2O. To avoid undesirable ammonia emissions, a so-called Ammonia Slip Catalyst, hereinafter ASC catalyst, is used downstream of the SCR catalytic converter to process any residual ammonia.
The function of the ASC catalyst depends in part on the temperature of the exhaust gases in such a way that if ammonia is oxidized in the ASC catalyst under conditions where the temperature in the ASC is high and the mixture is favorable, mainly NOX will be produced. On the other hand, if ammonia is oxidized under conditions where the conditions in the ASC are less favorable, N2O (nitrous oxide) will be produced instead. The ability of the SCR catalytic converter to store ammonia decreases with increasing temperature, with the result that ammonia will then preferably either leave the SCR catalytic converter or transition into N2O. As a result, an ASC catalyst normally acquires high levels of ammonia only at high temperatures. N2O emissions will thus depend on the temperature in both the SCR catalytic converter and the ASC catalyst. In the case where conditions are less favorable, the amount of ammonia and NOX will decrease downstream of the ASC catalyst, while N2O will be released. Because N2O is a gas that is a very powerful greenhouse gas, ca. 300 times stronger than carbon dioxide, it is desirable to reduce the emissions of N2O into the atmosphere.
Published patent application EP-2143901 describes a method intended to estimate the amount of N2O produced in vehicle exhaust gases. This is achieved in part by sensing the NOX concentrations upstream and downstream of the SCR catalytic converter. By then regulating the urea dosing based on these estimates, it is possible to thereby reduce the formation of nitrous oxide in the exhaust gas purification process.
U.S. Pat. No. 5,270,025 concerns a method for reducing nitrous oxide emissions while simultaneously reducing NOX. A combination of urea and an additional substance, such as glutamate, is used to regulate the emissions.
U.S. Pat. No. 5,547,650 describes an exhaust gas purification system in which N2O is removed by heating the exhaust gases and, finally, US-2009/0324453 describes a catalyst for NOX purification of exhaust gases by means of urea dosing.
A need thus exists to reduce emissions of nitrous oxide, and the object of the present invention is to achieve an improved exhaust gas aftertreatment system in which the emissions of N2O are reduced or avoided entirely.